The group of attorneys behind the lawsuit said that G4S and Salman were both aware of Mateen's mental instability and his intention to commit acts of violence.

“The evidence we’ve seen clearly shows there were multiple warning signs before the tragic Pulse nightclub shooting, and we believe those named in this complaint – the employer of Omar Mateen and his wife -- could have and should have taken steps to prevent this senseless act of violence,” Romanucci said.

G4S is the world’s largest security company, its headquarters are in Britain. G4S Secure Solutions, where Mateen was employed is based in Jupiter, Florida.

Salman currently is in jail awaiting trial. She has pleaded not guilty to federal charges of aiding and abetting, and obstruction of justice. Prosecutors have said Salman accompanied her husband when he cased locations for potential terrorist attacks and knew ahead of time that he was planning the attack.

"Rather than warn authorities, she kept it a secret and acted as his accomplice," Romanucci said.

Forty-nine people died in the worst mass shooting in recent U.S. history, and more than 50 people were injured.